In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material comprising chemical components that change colour or density by a physical or chemical process when the material is heated. A particular interesting direct thermal imaging element comprises an organic silver salt in combination with a reducing agent. When being heated, the silver ions are developed to metallic silver.
Image-wise heating can be obtained by means of a thermal printing head comprising an array of juxtaposed heating elements, e.g. resistors.
Most commonly line-wise recording heads are used. In this case the recording head comprises at least one linear array of heating elements, e.g. one resistor per pixel in a line. The thermal head writes one line at the time. A two-dimensional image is obtained by printing a number of parallel lines by transporting the thermal head and the recording material relative to each other in the so-called sub-scan direction, i.e. in a direction perpendicular to the line-wise printing direction of the thermal head.
The heating of the elements of the thermal head is controlled by an electric signal representation of the image. The density value of each pixel is represented by means of an N-bit digital signal value. Since the elements of a thermal recording head commonly are binary controllable devices, a time-multiplexing technique is applied for feeding an N-bit signal value to an element of the thermal head.
When printing an image, it is desired that a line printed by means of a linear array of heating elements is parallel with the edge of the recording material. To obtain this goal, the recording material which is transported in the printer is subjected to an alignment procedure prior to image recording.
Several alignment methods are known in the art.
In one prior art technique a sheet of recording material is completely taken out of a supply tray and is subjected to an alignment procedure before reaching the printing head. For example for aligning the sheet in a first direction use is made of gravity. The sheet which is first guided into a substantially vertical position is dropped onto a horizontal reference platen.
Alignment in a second direction perpendicular to the first direction is performed by means of a set of alignment stops. After alignment the sheet is transported towards the recording head. Since the sheet is entirely out of the supply tray during the alignment procedure the apparatus cannot be made very compact.
In an alternative alignment procedure the sheet of recording material is fed towards two (or more) pairs of touching rollers whereby the points of contact of the roller pairs define an alignment line. Once the sheet reaches the contact points between the pairs of rollers, the rollers are activated and the sheet is transported towards the recording head. The pairs of rollers are
commonly positioned behind the recording head in the direction of transport. As a consequence a large border of recording material cannot be used for printing.
This may be avoided by reversing the transport direction after alignment. However, this solution might soil the recording material and thus might decrease the accuracy of the printing process.